The present invention relates to an audio alarm which is to be installed on an entrance door, for example, to warn an unauthorized intrusion and includes an electrical switching arrangement which is only energized to turn the alarm on when any key other than a specified key is used.
A known alarm of this type has a circuit consisting of two switches, and is only actuated to render the alarm inoperative when the two switches are both placed in an "ON" position, thereby concluding the circuit. According to the known alarm, at least one of the two switches is used to place the alarm ready to signal a warning or on the alert, or off the alert. A number of different constructions have heretofore been known as switching means of placing the alarm on the alert or off the alert. A generally known switch has contacts simply placed apart, and is actuated when the contacts are electrically connected. The switching circuit is therefore easily interrupted by interposing an electrically insulated element between the two contacts. It will be easily understood that the known alarm has disadvantages since any person who knows the switching circuit arrangement may readily render the circuit or alarm inoperative by using his key so constructed.